Most heat transfer units are utilized in environments where erosion of the heat exchange tubes is of little consequence. This situation does not hold, however, in fluidized bed devices such as fluid catalytic crackers where high temperatures and the rapid movement of particles accelerate the erosion of the bed cooling tubes. Furthermore, the rupture of an eroded tube and release of water contained therein into a bed of hot catalyst at a temperature of 1200-1500.degree. F. creates rapid expansion and extremely high relieving loads for the vessel. To prolong tube life, the walls may be made heavier. However, heavier tubes are not only more expensive but are not as efficient in heat transfer. Furthermore, once erosion starts to take place on one part of a tube it often tends to accelerate quickly. Thus, individual tubes would likely be replaced on a regular basis. Replacement, however, requires closing down production and is thus extremely expensive.
In some portions of fluidized bed units other than the cooling tubes it has been common practice to provide erosion protection by attaching 3/4 inch thick hexagonal mesh in surrounding relation to the item, such as a valve, to be protected. The openings in the mesh are then filled with an abrasion resistant refractory. Such a covering would not be suitable for the conventional plain cooling tubes due to its large size and because it would interfere with heat transfer.